Hybrid braking systems include traditional hydraulic based brakes in addition to electric, brake-by-wire (BBW) brakes. The decoupling of the hydraulic and electric brakes allows electric brakes to apply normally even after the hydraulic brakes have faded. Application of the electric brakes includes a potential build-up of heat that may damage the brake calipers or drums. This heat build-up can be especially pronounced during an extended period of low braking levels, such as a mountain descent. During such an extended period, the brakes are deployed to control vehicle speed rather than to bring the vehicle to a stop.
BBW systems include a BBW electric actuator, such as a caliper or drum, including a mechanical unit that converts electrical energy to a rotor clamp force and an electric control unit (ECU) that controls the actuator. The mechanical and electrical units may or may not be in physical contact. In addition to the actuator, driver input sensors supply a signal indicating driver commands to the system—a pedal force sensor, a master cylinder pressure sensor and a pedal travel sensor.
Historically, the primary braking force control at low deceleration levels in a BBW system is pedal displacement based on the pedal travel sensor. For example, one function for calculating the command force function is:
Fcmd=Xpedal*fpedal(P)+Ppressure*fpressure(P), where f is a weighting function, xpedal is the pedal travel signal and Ppressure is the master cylinder pressure signal.
Use of pedal displacement as the control signal provides a robust signal with good bandwidth characteristics. Use of schemes of this type provide good pedal feel for the inherent system compliance, as pedal travel has been effectively used in full BBW systems to produce acceptable performance under all braking conditions. However, pedal displacement varies with vehicle build, use and age. Thus, in a hybrid system, the brake balance between front and rear can change due to changes in the pedal force (i.e. master cylinder pressure) to travel signal relationship. These changes may adversely affect front/rear brake balance. In an attempt to correct the front/rear brake balance, some systems utilize wheel speed data during braking events to maintain front/rear brake balance, in a process referred to as Dynamic Rear Proportioning (DRP).
However, measurements of hybrid brake system vehicle deceleration level and fluid pressure in both front calipers, during an extended braking event, reveal variation in relationship between pedal displacement and master cylinder pressure. FIGS. 1, 2 and 3 illustrate this relationship, and illustrate that after the first 100 seconds of the braking event the relationship between pedal travel and master cylinder pressure changes. This change illustrates that the relative magnitude and rate of change are system dependent and may have considerable variation.
FIG. 3 illustrates similar behavior, but with higher torque levels, simulating an increased downhill grade during the braking event or increased vehicle mass. As shown, the pressure increase as the driver compensates for reduced rear braking resulting from pedal travel affecting the rear actuator.
In an extended braking situation, reliance on wheel speed can lead to excessive heat build up in the rear actuator. In contrast to transient events, for example, parking lot maneuvers, utilizing pedal travel while at low speeds as the dominant input produces an undesirable shift in brake balance, as the signal resolution for DRP is limited. FIG. 4 illustrates increasing front bias. However, a greater concern is rear bias, as in such a situation, the rear actuator would produce more torque under the same deceleration condition—increasing the heat load on the electric systems of the rear brakes and potentially causing undesirable performance of the rear brakes.
Therefore, it would be desirable to provide a method of balancing front and rear brake forces in a hybrid brake system that overcomes the aforementioned and other disadvantages.